Typical solid electrolytic capacitors of the prior art have the construction shown in FIG. 14. A capacitor element 15 comprises a generally rectangular parallelepipedal anode body 1, and an anode leading member 16 in the form of a rod and embedded in the anode body 1. The anode body 1 is a sintered body of valve metal (such as tantalum, niobium, titanium, aluminum or the like). The anode body 1 has a dielectric coating layer 2 formed over the surface thereof by oxidizing. Formed over the dielectric coating layer 2 is a solid electrolyte layer 3, which in turn has a cathode leading layer 4 formed thereon. The solid electrolyte layer 3 is made from an electrically conductive inorganic material such as manganese dioxide, or an electrically conductive organic material such as a TCNQ complex salt or electrically conductive polymer. The cathode leading layer 4 is made, for example, from carbon or silver.
A platelike anode terminal member 61 is joined to the anode leading member 16 by resistance welding. A platelike cathode terminal member 62 is joined to the cathode leading layer 4 using an electrically conductive adhesive 5. The capacitor element 15 is covered with a packaging resin portion 7 having a substantially rectangular parallelepipedal configuration. The anode terminal member 61 and the cathode terminal member 62 partly extend through the resin portion 7 to the outside, and are bent downward along the outer surface of the packaging resin portion 7. The anode terminal member 61 and the cathode terminal member 62 each have an outer end portion disposed along the lower side of the resin portion 7 and soldered to a mount base board.
When the anode and the cathode of such a solid electrolytic capacitor short-circuit, for example, due to a break of or damage to the dielectric coating layer 2, self-generation of heat occurs, possibly producing fumes or a fire in an extreme case. In the case where the solid electrolyte layer 3 is made from a conductive inorganic material, the layer 3 is less likely to undergo healing until becoming heated to a considerably high temperature even in the event of self-generation of heat. Further when the conductive inorganic material contains oxygen, the solid electrolyte layer is prone to fuming or ignition. With solid electrolytic capacitors wherein a conductive inorganic material is used for the solid electrolyte layer 3, it is therefore practice to provide between the cathode leading layer 4 and the cathode terminal member 62 a fuse which is irreversibly breakable with overcurrent or excessive heat as a countermeasure against short-circuit current (see the publication of JP-A No. 1994-20891).
As a countermeasure against short-circuiting, it is also practice to provide a current control layer which is reversibly increasable in electrical resistance with overcurrent or excessive heat, between the cathode leading layer 4 of the solid electrolytic capacitor and the cathode terminal member 62 thereof (see the publication of JP-A No 1997-129520). This current control layer is made from an insulating polymer having electrically conductive particles admixed therewith. The layer is low in resistance value at room temperature owing to many conductive paths produced by the contact of conductive particles, whereas when heated to a high temperature, the layer exhibits a high resistance value due to the expansion of the insulating polymer which diminishes the conductive paths. In the event of a rise in temperature due to short-circuit current, therefore, the short-circuit current flowing through the solid electrolytic capacitor is restricted to a very small value. Ceramic capacitors are also known which have a current control element serving as a countermeasure against short-circuiting and comprising such a current control layer as sandwiched between metal plates (see the publication of JP-A No. 1999-176695).
One of the features of solid electrolytic capacitors is being low in equivalent series resistance (ESR). With electronic devices adapted to exhibit higher performance, it is desired that solid electrolytic capacitors be further lower in ESR. Widely used in recent years are solid electrolytic capacitors which comprise a solid electrolyte layer 3 made from a conductive polymer which is 10 to 100 times as high as the manganese dioxide in conductivity.
However, if the solid electrolytic capacitor is provided with a fuse serving as a countermeasure against short-circuiting as described above, the fuse which is none other than a resistor gives greatly increased ESR to the capacitor. It is therefore impossible to provide a fuse in solid electrolytic capacitors which are designed for reduced ESR, especially in those wherein a conductive polymer is used.
The fuse provided in solid electrolytic capacitors has another problem in that it is difficult to serve as a countermeasure against fuming and ignition due to a moderate rise in temperature although capable of serving such a function against instantaneous overcurrent. In order to prevent fuming and ignition due to such a rise in temperature, the solid electrolytic capacitor must be provided with current control means which functions at a temperature lower than the melting point (200 to 300° C.) of usual fuses, preferably at 100 to 150° C. The packaging resin portion 7 of the solid electrolytic capacitor is prepared by heating, for example, a solid epoxy resin at about 180° C. for melting, pouring the molten resin into a mold and thereafter holding the mold at the same temperature for several minutes for thermal curing. Accordingly, if an irreversible element such as a fuse of low melting point is used as current control means for the solid electrolytic capacitor, the current control means will melt in the course of fabrication of the packaging resin portion 7 of the capacitor.
The conventional current control layer for use in solid electrolytic capacitors as disclosed in the publication of JP-A No. 1997-129520 undergoes healing at a high temperature of over 300° C. and is therefore unsuitable for preventing fuming and ignition due to a moderate rise in temperature. Furthermore, if the current control layer is formed directly between the cathode leading layer 4 and the cathode terminal member 62 in the process for fabricating the solid electrolytic capacitor, the process will then require a prolonged period of time and become more cumbersome. Additionally, experiments conducted by the present inventor have revealed that solid electrolytic capacitors having such a current control layer are comparable in ESR to those having a fuse. Presently, solid electrolytic capacitors having low ESR and comprising a current control layer still remain to be realized.
The present invention, which overcomes the above problems, provides a solid electrolytic capacitor having low ESR and comprising a current control layer, and more particularly a solid electrolytic capacitor which comprises a current control layer and which is adapted to control current at a lower temperature than conventional solid electrolytic capacitors, the capacitor being capable of controlling current after fabrication even if exposed to a high temperature during the process of fabrication.